My invention relates broadly to processes involving the removal of contaminants from pit wastes. More particularly, the present invention relates to a process for the treatment of waste products generated by wood preserving plants that employ chromated copper arsenate (CCA).
Chromated copper arsenate is used as a wood preservative in a variety of processes. Wood products impregnated with Chromated Copper Arsenate solutions are resistant to decay and deterioration, and they are substantially immune from moisture and water induced degradation. CCA-treated wood products are resistant to decay when the wood is in contact with the ground or water. Pit wastes and byproducts from the process are also resistant to water, making it difficult to treat them for environmentally safe disposal.
In conventional CCA pressure treating processes, wood products are inserted into a pressure vessel that is then flooded with a liquid chromated copper arsenate solution and pressurized for a predetermined time. The CCA solution is eventually forced into the pores of the wood. The unused solution is thereafter pumped from the vessel. A partial vacuum is then induced in the vessel to remove excess CCA solution from the wood. Wood products removed from the vessel are allowed to drip-dry for a predetermined time. The drippings from freshly treated wood products are collected by a curbed concrete slab and routed to a concrete pit. There the drippings are filtered, and wastes are screened out. The recovered CCA-bearing liquid is returned to the preserving process for further use.
Dirt, concrete particles, wood and various other materials build up in the pit. Pit wastes collected during chromated copper arsenate (CCA) wood treatment comprise a mixture of dirt, pieces of wood, and miscellaneous organic and inorganic substances. These pit wastes contain chromated copper arsenate, and they must be removed from the pit from time to time. Presently, these waste products are removed to hazardous waste landfills by the manufacturers of chromated copper arsenate. These wastes are stored in fifty-five gallon drums, and at the present rate of wood treatment there are approximately 5,000 such drums stored in hazardous waste landfills each year in the United States.
It is well known in the art to leach arsenic bearing compounds and dissolve them with sulfuric acid. The adjustment of pH can occur with a variety of bases and acids. Recycling material back in to a parent process is also well known in the art.
Numerous U.S. patents deal with neutralization or disposal of the elemental components used in the aforedescribed process. U.S. Pat. No. 4,244,927 teaches the recovery of an arsenic compound involving "leaching" and liquid-solid separations and filtering. This method employs sulfuric acid as a leaching agent.
U.S. Pat. No. 5,141,753 also teaches leaching and neutralization of metallic substances with sulfuric acid. It is specifically directed to a copper-arsenic precipitate produced from refining copper during the production of arsenic bearing preservatives for wood treating. Neutralization occurs with lye.
U.S. Pat. No. 4,532,044 discloses a process that removes arsenic fluids with spent oil shale. U.S. Pat. No. 4,948,516 shows a method of disposing of wastes wherein arsenate is involved. U.S. Pat. No. 5,137,640 shows the recovery of arsenic precipitates from solutions of arsenic salts and sulfuric acid. U.S. Pat. No. 4,241,039 discloses a method of removing arsenic, wherein sulfuric acid is also used as a preliminary reagent. U.S. Pat. No. 4,240,826 is a very complicated process for handling arsenic. In U.S. Pat. No. 3,911,078 arsenic compounds are precipitated out of a reaction with sulfuric acid.
Chemical leaching and arsenic solutions are disclosed in U.S. Pat. Nos. 4,610,723, 5,024,769, and 4,244,735. U.S. Pat. No. 4,369,164 shows the recovery of copper and arsenic from solutions. U.S. Pat. No. 5,045,115 shows a system for treating solid waste residues. U.S. Pat. No. 5,106,510 shows the combined filtration and dissolving of metals. U.S. Pat. No. 3,948,802 shows a method of recycling and saving materials during wood processing. U.S. Pat. No. 4,526,687 shows how pit waste treatments can involve recirculation and recovery of materials.
The known prior art simply fails to disclose a workable process to remove chromated copper arsenate from contaminated solids. It is thus desirable to provide a process for decontaminating pit waste cellulosic products impregnated with chromated copper arsenate.
Hence, it is desirous to provide a process wherein pit waste from the CCA treatment of wood can be reduced to non-hazardous waste material that is fit for environmentally safe disposal. It is further desired that the chromated copper arsenate reclaimed for reuse. Such a process must be amenable to the treatment of soils or other solids contaminated with chromated copper arsenate resulting from accidental spills. The process should yield material having less than five parts per million of elemental compounds containing chromium, copper, or arsenic.